Ferrite circulators have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications A waveguide circulator may be implemented in a variety of applications, including but not limited to low noise amplifier (LNA) redundancy switches, T/R modules, isolators for high power sources, and switch matrices. One important application for such waveguide circulators is in space, especially in satellites where extreme reliability is essential and where size and weight are very important. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices. In most of the applications for waveguide switching and non-switching circulators, small size, low mass, and low insertion loss are significant qualities.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120° and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, a gyromagnetic effect is created that can be used for switching the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of switching between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. Radio frequency (RF) energy can be routed with low insertion loss from one waveguide arm to either of the two output arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction.
Generally, these three-port waveguide switching circulators are impedance matched to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conventional three-port waveguide switching circulators typically have one or more stages of quarter-wave dielectric transformer structures for purposes of impedance matching the ferrite element to the waveguide interface. The dielectric transformers are typically used to match the lower impedance of the ferrite element to the higher impedance of the air-filled waveguide so as to produce low loss.
Previous patents (U.S. Pat. No. 4,697,158; U.S. Pat. No. 3,277,399; U.S. Pat. No. 4,058,780, Pub. No. WO 02/067361 A1) have described approaches for achieving broad bandwidth through the additional of impedance matching elements. Broadband circulators have high isolation and return loss and low insertion loss over a wide frequency band, which is desirable so that the circulator is not the limiting component in the frequency bandwidth of a system. Broad bandwidth also allows a single design to be reused in different applications, thereby providing a cost savings. These prior art approaches for achieving broad bandwidth generally involve the additional of quarter-wave dielectric transformers or steps in the height or width of the waveguide structure to thus achieve impedance matching the ferrite element to the waveguide port. For example, U.S. Pat. No. 4,697,158 discloses achieving impedance matching by providing a step or transition in the waveguide pathway. This technique eliminates the standard dielectric transformers, but is very sensitive to dimensional variations, resulting in a design that is difficult and expensive to manufacture reliably. This design also relies on the presence of a significant gap or spacing between adjacent ferrite elements, increasing the size and weight of the structure. These methods all require impedance matching elements in addition to the ferrite element in order to achieve acceptable performance. Other patents, such as U.S. Pat. No. 5,724,010, discuss changing the shape of the ferrite resonant structure to achieve broadband performance. However, these ferrite structures are restricted to fixed-bias applications with a single direction of circulation.
Referring now to FIG. 1, there is shown a top view of a conventional ferrite element. Although magnetizing windings are not shown, dashed lines 135 denote the apertures for the magnetizing windings. Apertures 135 for the magnetizing windings may be created by boring a hole through each leg of the ferrite element, for example. If a magnetizing winding is inserted through the apertures, then a magnetizing field may be established in the ferrite element, as would be evident to those possessing an ordinary skill in the pertinent arts. The polarity of this field may be switched, alternately, by the application of current on the magnetizing winding to thereby create the switchable circulator.
Resonant section 130 exists where the legs of device 101 converge inside the three apertures 135. As would be evident to those possessing an ordinary skill in the pertinent arts, the dimensions of resonant section 130 determine the operating frequency for circulation in accordance with conventional design and theory. The sections 140 of the ferrite element in the area outside of the magnetizing winding apertures 135 may act as return paths for the bias fields in the resonant section 130 and as impedance transformers out of the resonant section. Faces 150 of the ferrite element are located at the outer edges of the three legs.
Referring now to FIG. 2, there is shown a top view of a conventional single-junction waveguide circulator structure. FIG. 2 shows a ferrite element 101 with a quarter-wave dielectric transformer 103 attached to each leg. A filler material 102 may be disposed on the top and bottom surfaces of ferrite element 101. Filler material 102 may be used to properly position the ferrite element in the housing and to provide a thermal path out of ferrite element 101, which may be necessary for high power applications. Conventional circulators have minimized the diameter of this spacer for impedance matching purposes, and the diameter is generally smaller than the size of resonant section 130 discussed hereinabove. An empirical matching element 104 may be disposed in close proximity to the quarter-wave dielectric transformers 103.
The conventional components described above may be disposed within the conductive waveguide structure 100, which is generally air-filled. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conductive waveguide structure 100 may include waveguide input/output ports 105 as discussed above. Ports 105 may provide interfaces, such as for signal input and output, for example. Empirical matching elements 104 may be disposed on the surface of conductive waveguide structure 100 to affect the performance. Matching elements 104 may be capacitive/inductive dielectric or metallic buttons that are used to empirically improve the impedance match over the desired operating frequency band.
Referring now to FIG. 3, there is shown a partial side view of a conventional single-junction waveguide circulator structure. As may be seen in FIG. 3, only one of the three legs of the ferrite element is shown. This view shows filler materials 102 located between the walls of waveguide structure 100 and ferrite element 101. As a result of filler materials 102 being smaller in diameter than the legs of ferrite element 101, air gaps 110 exist above and below portions of the legs of the ferrite element. Air gaps 110 may be approximately one-third the height of the waveguide in the E-plane axis. Air gaps 110 in the E-plane may be prone to high peak power breakdown effects such as arcing or multipactor, as would be evident to those possessing an ordinary skill in the pertinent arts. Thus, air gaps 110 may limit the maximum peak power handling capabilities of conventional circulator designs.
Accordingly, a need exits for a device that improves peak power handling, heat dissipation, and other characteristics, in part by elimination of a gap adjacent to the conductive portion of a waveguide.